Can Lack of Fusion convert into crack during welding

Date 12-07-2018 12:35 Edited 12-07-2018 13:05

Hey Mayur,
Not from subsequent multi pass welds.
Additional passes does not exert tensile stresses perpendicular to the lack of fusion line.

No literature to back up my 2 cents.

Tyrone

Edit: Just read your other post. (Crack looks like it started from lack of fusion).
I think you may have multiple factors that caused the crack. The only way to know for sure is to do a fractography analysis.

By 803056

Date 12-08-2018 14:46

The residual stress resulting from the thermal cycle is in tension both in the longitudinal and transverse directions.

I view the situation as the residual tensile stress transverse to the axis of the weld is like a length of rope extending from one side of the joint to the other. The length of rope is loaded in tension. The incomplete fusion with its primary axis parallel to the axis of the weld is like a knife cut across the length of rope.

Now, I ask you, what is the influence of the knife cut across the length of rope? As the tensile load on the rope increases, so does the influence of the knife cut.

Al

IncompleteFusiontheinfluenceof8-Dec-2018.jpg (67k)

By Mayur

Date 12-10-2018 03:31

Thank you very much.

That means my perception is right that present lack of fusion defect was extended as crack in weld metal due to residual welding stresses.

Dear expert, I shall be very grateful to you if you can share me any literature, article, part of any handbook related to the same situation.

Thanks & Regards,
Mayur

By Jim Hughes (***)

Date 12-10-2018 13:30

Mayur,
There is some good resources out there for you to review that could very much help with your issue. Having SME's perform failure analysis on the component to determine failure. I can recommend a couple of companies we use off line if you like. Second, purchasing ASM's (American Society for Metals) "Elements of Metallurgy and Engineering" resource books. They have others also that might help. They have a web site and book store. asminternational.org Hope that helps.

By Mayur

Date 12-10-2018 14:57

Dear Expert,

Where did you find above mentioned figure? Did you make this figure by yourself? or have you taken this figure from any resource literature?

By 803056

Date 12-12-2018 03:28

It is a sketch I drew to help you visualize how the incomplete fusion is transverse to the tensile forces (residual stress) transverse to the longitudinal axis of the weld. The incomplete fusion is a significant stress riser. The larger it is, the grater the influence. It is a stress riser, but it also represents a reduction is cross section. That increases the unit stress in that localized area,

If you isolate the area where the incomplete fusion is located, it is easy to visualize how it reduces the cross-sectional area. The stress, S, divided by the cross section equals the unit stress. When the unit stress exceeds the tensile stress of the base metal/ weld metal, the crack is going to increase in size, and increase the unit stress even more. The incomplete fusion is going to "open “and the unit stress increases and shed some of the load to adjacent areas.

I'm trying to help you envision the mechanics of what happen and how if the incomplete fusion is small, it may be "insignificant. However, as the size of the incomplete fusion increases in size, it can represent a significant reduction in cross section. At this point the loading is static.

If the loading is cyclic, the repercussions are more significant.

Best regards - Al

By Tyrone

Date 12-12-2018 12:35 Edited 12-12-2018 12:40

Hey Al,
Here's what I envisioned what is going on during multi pass welds when there's lack of fusion on the first pass. See crude/crappy sketch.

I see a small tensile stress component acting perpendicular to the lack of fusion. That's why I think there are more factors that caused this crack than just shrinkage stresses due to subsequent weld passes.

If lack of fusion on first past yield results in cracking, we would advocate for zero allowable for LOF.

Tyrone

By 803056

Date 12-16-2018 00:43

No argument there. Whether the incomplete fusion is in the root, intermediate, or cover beads is immaterial. The transverse tensile stress (residual stress) is a major contributing factor in initiating and propagating the longitudinal crack.

The incomplete fusion acts as a stress intensifier with sharp end conditions, its linear (length is several times the width), and if the IF is in the root bead or cover bead, it is surface breaking where the unit stresses tend to be higher. Especially a problem if there are bending stresses transverse to the longitudinal axis of the weld.

Merry Christmas - Al